1. Field of the Invention
The present invention relates to methods of writing composite 1-step holograms.
2. Description of Related Art
PCT applications WO01/45943 and WO01/42861 (Brotherton-Ratcliffe et al.) described a holographic printing system based on a rapid pulsed laser. This system was able to operate in two modes (“a dual-mode printer”). In the first mode, a holographic master hologram, also known as an H1, is produced. This hologram is then copied conventionally to a second and final hologram, known as the H2, by machines similar to those described by Grichine et al. (1997). In the second mode of operation, the final hologram is written directly.
In its first mode of operation, the digital data required by the system disclosed in WO01/45943 and WO01/42861 consists of ordinary perspective views that are easily generated by standard 3D commercial software packages. These images are then trivially distorted in order to compensate for inherent optical distortion that is present in the invention.
In its second mode of operation, the digital data required by the system disclosed in WO01/45943 and WO00/42861 are derived by applying various general mathematical transformations on the ensemble of the undistorted data-set that would be used for the generation of a hologram under the first mode of operation.
U.S. Pat. No. 5,793,503 (Haines et al.) described various transformations for the preparation of digital data for writing the hologram of a 3D computerized model using an ideal hologram printer. This arrangement concentrated on the treatment of data from a specialized 3D model but also treated the case of the generation of the required digital data from the conventional perspective views produced by standard commercial software.
The methods described in U.S. Pat. No. 5,793,503 are inappropriate for application to the type of holographic printers described in PCT applications WO01/45943 and WO01/42861. One reason for this is that such printers intrinsically possess very large optical distortion caused by a finite 5th order Seidel coefficient in the writing objective. This distortion, which generally varies from pixel to pixel in the case of non-static SLM printers should not be corrected for independently by sequential application of correction algorithms as this would lead to both image noise and computational disadvantage.
Another reason why the arrangement disclosed in U.S. Pat. No. 5,793,503 is inappropriate in the present context is that the 1-step holograms that are produced by the system disclosed in PCT applications WO01/45943 and WO01/42861 must, on display, generally be illuminated by a light from a point source which does not correspond geometrically to the recording illumination used within the printer and thus any proper method, for transforming perspective data into the required data, should be based on a general diffraction model. In its generality such a model must take into account such parameters as the emulsion and hologram substrate refractive indices as well as the recording and ray replay geometry.
Although the system disclosed in WO01/45943 and WO01/42861 represents a considerable advance over the prior-art it too suffers from various limitations. In particular, by not integrating the correction for optical distortion of the objective, into the data rearrangement transforms necessary for writing 1-step holograms, image quality is inevitably compromised. In addition, by only tracking the reference recording beam in one dimension, rather than two, the printer is fundamentally unable to produce large format holograms that are illuminated by close point-source lights. Finally, by seeking to correct for geometrical image distortions alone the prior-art printer suffers from increasing discoloration effects as a closer (and more realistic) point-source illumination is demanded.